The invention relates to a keyboard of the kind comprising two transverse arrays of conductors defining a number of rows and columns of cross-points at each of which a key is positioned whereby depression of a key causes the conductors at the corresponding cross-points to be electrically connected; and a scanning device connected to each of the conductors and adapted during a scanning operation to apply a high or low potential to all the conductors of the two arrays and subsequently sequentially to apply a potential of the opposite type to each conductor of one array and to monitor the potential of the conductors of the other array such that if the potential of a conductor of the other array changes, this indicates that the key at the corresponding cross-point is depressed, wherein the scanning device is connected directly to each of the conductors of the arrays, and wherein an indicating device is connected between one of the row conductors and a power supply, the condition of the indicating device being determined in accordance with the potential on the row conductor. Such keyboards are hereinafter referred to as of the kind described. It should be understood that the reference to rows and columns does not imply that these are necessarily orthogonal.
An example of a keyboard of the kind described is illustrated in DE-A-3640432.
In order to scan the keyboard, all the conductors are initially raised to a high potential and then each row conductor is pulsed in sequence at the low potential and the column conductors are simultaneously monitored to determine if any senses the change in potential in the row. If it does, this indicates that the key at the corresponding cross-point has been depressed.
The indicating device or devices which are used in keyboards of the kind described are activated by suitably setting the potential of the associated row conductor and are used to indicate conditions of the keyboard such as the scroll condition, number lock, capital (uppercase) lock and the like. During the scanning cycle, since the potential on the associated row conductor is changed, this will cause the condition of the indicating device to change also but only for a short time which is generally acceptable and will not be noticed in practice. However, a problem arises if two adjacent keys are inadvertently depressed simultaneously. In this case, if these are in the same column and adjacent rows, the condition of the indicating device will be changed not only during the scanning of the row to which the device is connected but also when the other row is scanned. This is undesirable.